The phosphorus (P) concentration plays an important role in the eutrophication of the marine environment and aquatic systems. Phosphorus adsorption/desorption experiments were carried out in a flume channel in the hydraulics laboratory at Cardiff University. The experiments were carried out for three types of sediments such as sand, silt and mud, from two different estuaries in South Wales. The sediments were exposed to artificial phosphorus contamination, and then placed in the flume channel with controlled hydraulic characteristics. Five scenarios were studied and P concentrations were monitored in the sediments at time intervals ranging from zero to over 72 hr. The aim of this study was to improve on our knowledge of the phosphorus-sediment interactions in the estuarine environments and in relation to tidal cycle effects. The results of the 3-D numerical model TRIVAST were set-up to simulate the effects of the sediment size-phosphorus interactions in the flume channel. The model results showed good agreement with the collected data.